1. Field of the Invention
The present invention relates to a drill, and particularly to a drill in which the cutting resistance can be reduced during a cutting process while improving the tool life.
2. Description of the Invention
In order to reduce the cutting resistance during a cutting process, a thinning has conventionally been formed at the tip of a drill as, for example, a twist drill that is disclosed in Japanese Patent Application Laid-Open No. 2000-271811. According to the twist drill, the length of the chisel edge is shortened by having a thinning 5, and a contact area of the tip of the twist drill with a workpiece is accordingly reduced during a cutting process, which results in reduction of the cutting resistance.
Incidentally, according to the above-described twist drill, the cutting resistance can be reduced in general as a rake angle α of a thinning edge 6 that is formed by the thinning 5 is increased. On the other hand, when the rake angle is large, the thinning edge 6 is likely to be chipped due to reduction of the strength thereof.
Consequently, in the twist drill, the rake angle α of the thinning edge 6 is set in a range of 5 to 15 degrees so as to improve the tool life.
However, in the case where the rake angle α of the thinning edge 6 is set in consideration of the tool life as the above-described twist drill, there has been a problem that the cutting resistance can not be sufficiently reduced and it is difficult to satisfy both of the improvement of the tool life and the reduction of the cutting resistance.